In an early form of optical fiber gyroscope, a light beam is split into two components that pass in opposite directions through an optical fiber sensing coil. The component that passes in the direction of rotation of the coil takes a longer time to traverse the coil and is delayed in phase with respect to the other component. The two light components are combined again on a photodetector, and produce an amount of light dependent upon their phase difference, which is dependent upon rate of rotation of the coil. In a more advanced, phase nulling optical gyroscope, a small additional frequency is added to or subtracted from one of the light components. After traversing the coil, both light components at corresponding ends of the coil, have a predetermined constant phase relationship regardless of the rate of rotation of the coil because of the frequency shift. A ramp voltage applied to a phase shifter creates the additional frequency, and the slope of the ramp and the frequency of its resetting indicate the amount of increased frequency that had to be added to or subtracted from one light component, and therefore indicate the rate of rotation of the coil.
The detection of coil rotation can be a problem when the rotation rate is very small. A very small rotation is common, as where the vehicle on which the optical gyroscope is mounted maintains an essentially fixed orientation along a flight path. Then the ramp may have a very low resetting rate. Furthermore, the phase shifter to which the ramp voltage is applied, may not have a linear relationship between the rate of voltage increase and the frequency change. An apparatus which could produce a more precise and accurate indication of coil rotation, and in a digital form, especially for very low rates of coil rotation, would be of considerable value.